Keep image picker previews sharp on HiDPI displays

shell/plugins/image-picker/ImagePicker.qml

@@ -603,13 +603,15 @@ Item {
                 Image {
                   id: image
                   anchors.fill: parent
-                  // Even an uncached 6K wallpaper decodes at card size, off the
-                  // GUI thread. Departing cards release their images instead of
-                  // retaining every preview visited in a large collection.
+                  // Decode at the expanded card's physical size, off the GUI
+                  // thread. Keep that size during navigation to avoid reloads.
+                  // Departing cards release their images instead of retaining
+                  // every preview visited in a large collection.
                   // Queue the selected preview first; neighbors must not delay
                   // the image the user opened the picker to see.
                   source: (item.selected || root.neighborImagesEnabled) && item.thumbnailPath ? Util.fileUrl(item.thumbnailPath) : ""
-                  sourceSize: Qt.size(root.expandedWidth, root.expandedHeight)
+                  sourceSize.width: Math.ceil(root.expandedWidth * Screen.devicePixelRatio)
+                  sourceSize.height: Math.ceil(root.expandedHeight * Screen.devicePixelRatio)
                   fillMode: Image.PreserveAspectCrop
                   asynchronous: true
                   cache: false

test/shell.d/image-picker-test.sh

@@ -151,10 +151,24 @@ assert(
 )
 assert(
   /model: visibleImages/.test(imagePickerQml) &&
-    /sourceSize: Qt\.size\(root\.expandedWidth, root\.expandedHeight\)/.test(imagePickerQml) &&
     /asynchronous: true\s*cache: false/.test(imagePickerQml),
   'image picker renders its window with bounded asynchronous decoding'
 )
+const sourceWidth = imagePickerQml.match(/sourceSize\.width: ([^\n]+)/)[1]
+const sourceHeight = imagePickerQml.match(/sourceSize\.height: ([^\n]+)/)[1]
+const decodeSize = new Function('root', 'Screen', `return [${sourceWidth}, ${sourceHeight}]`)
+for (const [scale, expected] of [
+  [1, [768, 475]],
+  [1.25, [960, 594]],
+  [1.5, [1152, 713]],
+  [2, [1536, 950]]
+]) {
+  assertDeepEqual(
+    decodeSize({ expandedWidth: 768, expandedHeight: 475 }, { devicePixelRatio: scale }),
+    expected,
+    `image picker decodes enough physical pixels at ${scale}x display scale`
+  )
+}
 assert(
   imagePickerQml.includes('(item.selected || root.neighborImagesEnabled)') &&
     /onStatusChanged: if \(item.selected && \(status === Image.Ready \|\| status === Image.Error\)\) root.neighborImagesEnabled = true/.test(imagePickerQml),